1. Field of the Invention
This invention relates in general to diverters and blowout preventer systems for drilling rigs. In particular, the invention relates to diverter and blowout preventer systems and methods for use with bottom supported offshore drilling rigs.
2. Description of the Prior Art
Diverter systems for bottom supported offshore drilling rigs are known in which a diverter element is provided in the support housing attached to the support beams beneath the drilling rig rotary table. Such diverter systems have provided for a vent line and a flow line in the permanent housing beneath the rotary table. Such systems have required external valve systems in the vent line to assure that when the diverter in the permanent housing opens the fluid system to the vent line, the flow may be directed away from the drilling rig. In such prior art systems, a spacer spool has been typically provided beneath the support housing and a thirty (30) inch overshot connection has been provided between the spacer spool and the thirty (30) inch outside diameter drive pipe or structural casing.
Fatal and costly accidents have resulted from the complexity of prior art diverter systems described above. Typical prior art diverter systems have included an annulus closing device, external vent and flow line valves, actuators, limit switches and sequenced controls. This complicated valving and piping of the prior art has been further complicated by the inherent risks of manipulating loose packer inserts into the diverter itself. The complexity of the prior art systems has invited a variety of human error and equipment malfunctions.
One problem with the prior art systems has involved the use of external valving in the diverter system. Valves which are external to the diverter unit not only add clutter to the diverter system and the rig configuration, but also require multiple control functions which are required to operate properly. For example, the prior art diverter system valves have required an actuating pressure signal that is regulated to a discrete pressure level different from the operating pressure level of the diverter unit. The need for separate and different control functions executed in only one safe sequence has required separate pressure regulators and connecting components that are in different locations on the underside of the rig floor. Such a requirement has invited mistakes and malfunctions.
In addition to the problem of multiple control functions, there has existed problems with crossed connections in prior art diverter systems. Misconnection of control lines can cause a valve to be closed when it should be open which could result in an explosion in the diverter or breach of the casing.
Another problem of the prior art diverter systems has been exposure of delicate parts such as hydraulic tubing and fittings, limit switches, mechanical linkages and valve actuators to the rig work area. Such exposure has in the past caused breakage and damage to such parts. System malfunctions which result from damage to exposure can be catastrophic.
Another problem of prior art diverter systems has been the result of vent line blockage. Because the vent valve has been remote from the diverter unit itself, a stagnant space has existed at a critical location in the vent line. Buildup of solids and caking of mud in such a dead space may cause the critically important vent line to be choked off. A restricted or shut-off vent line may cause a dangerous pressure increase while being called upon to divert.
Still another problem of prior art diverter systems has involved the use of component sources from a number of different manufacturers. The annulus closing device, vent and flow line valves, actuators, sequencing devices and control system components have typically been provided by different manufacturers. Rig operating personnel are usually burdened with devising the vent line valve circuit interconnecting the components (which are often widely physically separated when installed) and stocking a varied assortment of spare parts using extraordinary caution to avoid misconnections and keeping a number of rig personnel trained to operate and maintain a diverse assortment of complicated components.
Still another problem of prior art diverter systems for bottom supported rigs has been the requirement of a high pressure valve in the vent line. Closure of such a valve has enabled the diverter unit to be converted to a blowout preventer after sufficient casing pressure integrity has been established. However, if this valve should inadvertently be closed during an attempt to divert, breach of the casing or explosion of the diverter system could threaten the safety of the rig itself.
Still another problem of prior art diverter systems has been the result of valve mismatch. While many different types of valves have been used in diverter systems, there has been no single valve that is especially well suited to the particular application of a diverter system. Selection of the type, size and rating of such valves has been a vexing puzzle for designers of rig valve systems which has been required to be solved usually when a new drilling rig is being built.
Another important disadvantage of the prior art diverter systems has been the necessity to stop drilling operations and manipulate packer inserts to facilitate annulus shut-off. Such a necessity has not only been a time consuming task, it has presented very real hazards. One such hazard has been the problem of forgotten inserts. Often in the course of determined efforts to drill ahead, fetching, installing and latching the packer insert is overlooked. Without such an insert there is no diverter protection. If the insert is in place, but not latched down in some prior art diverter systems, the packer insert is potentially a dangerous projectile.
A second problem resulting from the use of packer inserts has been the problem of an open hole hazard. There has been no protection from the insert type diverter against uncontrolled well fluid flows. Such lack of protection has left a serious safety gap in the drilling operation.
Still another problem of the use of packer inserts in the prior art diverter systems has been the problem of forgotten removal. If unlatch and removal of the packer insert has been inadvertently overlooked before pulling drill pipe from the hole, centralizers or the bottom hole assembly may be run into the insert, thereby endangering the drilling crew and equipment.
Still another problem of the use of packer inserts in the prior art drilling systems has been the problem of exploding packers. If during testing, the standard packer is not reinforced by an insert and/or a pipe in the hole, the hydraulic fluid pressure may cause the packer to explode, thus jeopardizing the safety of the crew.
Perhaps the most important problem of the prior art diverter systems has been the inherent risk of pressure testing in-situ. Pressure testing of prior art diverter systems has been accomplished by overriding the safety sequencing in the valves so that the vent line valve is closed simultaneously with closure of the annulus. Disastrous results have been experienced when the safety overriding mechanism has been unintentionally left in place when testing was complete and drilling was resumed.